Field of the Invention
The present invention relates to a clock recovery system, and more specifically to a clock recovery system for receiving MPEG-2 signals.
Related Art
The ISO Moving Picture Experts Group (MPEG) established a standard by which digital bit representations of audio, video and data sources can be encoded, transmitted, received, and reliably decoded. This standard has been published as document ITU-T H.222.0, entitled “Information Technology—Generic Coding of Moving Pictures and Associated Audio Information: Systems”. The MPEG-2 standard enables the transmission of digitized audio and video data in packetized form for insertion into a transport stream. Multiple sources of data may be multiplexed so that many users may share the same communications path.
In order to ensure reliable delivery of the audio, video and data to the end-user, the decoder clock must be synchronized with the encoder clock. Absence of synchronization will result in frame skips or frame holds in the case of video information, or its equivalent in the case of audio information. In order to achieve synchronization between the encoding clock and the decoding clock, the encoder inserts a time stamp into the transport stream, such a time stamp is known as a Program Clock Reference (PCR). The decoder uses these time stamps to ensure that the decoder time clock is synchronized to the encoder time clock and that the resulting transmission of audio, video and data is properly received and decoded.
In the traditional system infrastructure, if there are no PCR values carried within the stream, the decoder cannot evaluate the transport rate, it cannot lock on to the information stream, and the decoder clock will not be synchronized with the encoder clock.
What is needed is a more flexible approach to ensure the reliable decoding of single and multiplexed audio, video and data streams without the need to use PCR values. Further, it is desirable that the new approach require changes to firmware only and thereby be able to use existing PCR-based hardware.